Unravelling disequilibrium in basaltic crystal cargoes: an experimental and petrological study of mixing in primitive Icelandic magmas

大洋玄武岩构成了地球表面喷发的大部分物质，是熔体和晶体的复杂混合体，熔体在从地幔上升到岩浆室的过程中混合和结晶。不平衡记录，即两个或两个以上相互不平衡的相组合的并置，广泛保存在玄武岩晶体的结构和成分中。因此，真实的岩浆聚集和演化模型必须将不平衡的影响纳入其对岩浆过程的描述中。虽然相平衡研究已经为岩浆作用的本质提供了无数的见解，但利用平衡实验来理解自然样品中的不平衡特征的有效性仍然不清楚。因此，开展严格约束的不平衡实验以了解反应动力学是提高对天然玄武岩解释的下一步至关重要的一步。该项目的主要目的是定量地记录玄武岩岩浆的晶体含量如何响应由于岩浆混合而产生的化学和热不平衡。这将通过执行高压(3kbar)、高温(1160-1280°C)实验来实现，该实验旨在模拟冰岛特征良好的天然玄武岩中的混合过程。为了尽可能地分离化学和热不平衡的影响，一系列实验将研究含晶体和成分不均匀的原始玄武岩的等温和等压混合(开放系统化学不平衡)。第二个系列将研究冷却和加热对含有单晶的原始岩浆成分的影响，分别模拟卷吸和补给(闭合系统热不平衡)。不平衡实验将仅在已在先前项目中确定平衡相关系的起始组合物上进行。将对RUN产品进行成像(BSE)和分析，以量化晶体的织构(例如，晶体尺寸分布)和晶体对化学和热不平衡的成分响应。晶体生长、溶解和重新平衡的速度将通过在一系列时间尺度上进行实验来评估，这些时间尺度与冰岛火山系统喷发前动荡的时间尺度重叠。然后，新的实验数据将被用来重新评估冰岛一系列喷发的天然晶体中的不平衡特征，包括1784-1784年的Laki喷发和2014-2015年的Hluhraun喷发。例如，将研究混合对岩浆过程结晶记录产生偏差的能力。虽然该项目将专注于冰岛系统，因为它们具有良好的地质背景，但其发现将广泛与任何在其结晶货物中带有不平衡记录的玄武岩有关。

Oceanic basalts, which form the bulk of material erupted at the surface Earth, are complex hybrids of melts and crystals assembled during mixing and crystallisation of melts during ascent from the mantle and processing in magma chambers. Records of disequilibrium, i.e., the juxtaposition of two or more phase assemblages that are out of equilibrium with each other, are widely preserved in the textures and compositions of basaltic crystal cargoes. Realistic models of magma assembly and evolution must therefore incorporate the effects of disequilibrium into their descriptions of magmatic processes. Although phase equilibria studies have provided innumerable insights into the nature of magmatic processes, the validity of using equilibrium experiments to understand disequilibrium features in natural samples remains unclear. Carrying out well-constrained disequilibrium experiments to understand reaction kinetics is thus a crucial next step in improving the interpretation of natural basalts.The primary aim of this project is to document quantitatively how the crystal contents of basaltic magmas respond to chemical and thermal disequilibrium as a result of magma mixing. This will be achieved by performing high-pressure (3 kbar), high-temperature (1160-1280 °C) experiments designed to the mimic mixing processes that have been identified in well-characterised natural basalts from Iceland. In order to separate the effects of chemical and thermal disequilibrium as much as possible, one series of experiments will investigate the isothermal and isobaric mixing of crystal-bearing and compositionally heterogeneous primitive basalts (open system chemical disequilibrium). A second series will investigate the effect of cooling and heating on a single crystal-bearing primitive magma composition, simulating entrainment and recharge respectively (closed system thermal disequilibrium). Disequilibrium experiments will only be performed on starting compositions for which equilibrium phase relationships have already been determined in a previous project. Run products will be imaged (BSE) and analysed to quantify the textural (e.g. crystal size distribution) and compositional response of crystals to both chemical and thermal disequilibrium. Rates of crystal growth, dissolution and re-equilibration will be evaluated by performing experiments over a range of timescales that overlap with those of pre-eruptive unrest in Icelandic volcanic systems. The new experimental data will then be used to re-assess disequilibrium features in natural crystals from a selection of Icelandic eruptions including the 1784-1784 Laki and 2014-2015 Holuhraun eruptions. For example, the ability of mixing to bias the crystalline record of magmatic processes will be investigated. While this project will focus on Icelandic systems, because of their excellent geological context, its findings will be widely relevant to any basalts that carry records of disequilibrium in their crystal cargoes.